It has already been known to provide such a copying machine having a slit illumination system capable of projecting light beams issuing from a light source upon the document and further projecting the reflected light beams therefrom through a slit onto a photosensitive drum.
In FIG. 1, light passages in such conventional slit illumination system are shown only schematically. In this illumination system, part of the light beams delivered from exposure lamp 2a is condensingly reflected at a main elliptic mirror 3a and projected directly towards the surface of document las, while another part of the projected light beams from the exposure lamp is condensingly reflected once at a vice elliptic mirror 4a towards a plane type auxiliary mirror 5a and again reflected by the latter towards the same document surface las from the opposite side, with respect to the position of main elliptic mirror 3a and about the slit center Xa, for illumination of the document surface las in the mode of a slit. In this way, the illumination is carried out from both sides of slit center Xa of the illumination system. This illuminating arrangement and feature thereof are highly valuable, especially in the treatment of a book or the like voluminous document, by virtue of positive avoidance of shadow formation on the printed copies.
It should be noted that in such a slit illumination system as above referred to, the image focussing line of reflected light beams from main elliptic mirror 3a and that of another reflected light beams from vice elliptic mirror 4a are both above the glass-made, document table 1a. In this respect, references may preferably be had to FIG. 2(a) and FIG. 3(a), respectively. As a result, respective illumination intensity curves of the illuminated areas on the document surface las by reflected lights from the both mirrors 3a; 4a represent their peaks at small distances from the imaginary slit center plane Xa, upstream in this case therefrom, as shown in FIG. 2(b) and FIG. 3(b), respectively. Thus, the document has an illuminated slit-like area composed of these two illumination intensity curves, as shown in FIG. 4 drawn on a somewhat enlarged scale.
However, it is to be noted that the foregoing observation is only theoretical and of machine design aspect and, in practice, there are unavoidable machining and assemblying errors among exposure lamp 2a, main elliptic mirror 3a, vice elliptic mirror 4a, auxiliary mirror 5a and a slit member, not shown, serving for slit image formation and that these errors may give rise to a slight shift of the illuminated region as hinted by that of illumination intensity curve from L to L' relative to the slit center line or plane Xa in FIG. 5. Since the photosensitive drum, not shown, receives only slit-passed lights which are shown by a hatched area shown in the drawing, the very existence of said machining and assemblying errors will produce increase or decrease of projected light quantity on the drum, even among same model copying machines manufactured by a single manufacturing factory. As may be clearly seen, the now supposed case of FIG. 5 shows, however, a decrease in the projected light quantity only as an example thereof.